Wireless handheld or portable devices typically transmit and/or receive electromagnetic wave signals for one or more cellular communication standards and/or wireless connectivity standards and/or broadcast standards, each standard being allocated in one or more frequency bands, and said frequency bands being contained within one or more regions of the electromagnetic spectrum. For the transmission and/or reception of electromagnetic wave signals, a typical wireless handheld or portable device must include a radiating system capable of operating in one or more frequency bands with an acceptable radioelectric performance (such as for example in terms of input impedance level, impedance bandwidth, gain, efficiency, or radiation pattern). Moreover, the integration of the radiating system within the wireless handheld or portable device must be effective to ensure that the wireless device itself attains a good radioelectric performance (such as for example in terms of radiated power, received power, or sensitivity).
For a good wireless connection, high efficiency is further required. Another common design specification for the radiating system is the voltage standing wave ratio (VSWR) with respect to a typical 50 ohm impedance, which in case of for instance mobile phones, is typically expected to be below VSWR≦4, or preferably below VSWR≦3, and generally as close to VSWR=1 as possible.
In this text, the expression impedance bandwidth is to be interpreted as referring to a frequency region over which a wireless handheld or portable device and a radiating system comply with certain specifications, depending on the service for which the wireless device is adapted. For example, for a device adapted to transmit and receive signals of cellular communication standards, a radiating system having a relative impedance bandwidth of at least 5% (and more preferably not less than 8%, 10%, 15%, 20% or 30%) together with an efficiency of not less than 30% (advantageously not less than 40%, more advantageously not less than 50%) can be preferred. Also, an input return loss of 3 dB or better within the corresponding frequency region can be preferred.
Other demands for radiating systems to be integrated in wireless handheld or portable devices are focused on minimizing the size and the manufacturing costs. Hence, the radiating system is expected to be small for occupying as little space as possible in order to favor the integration of other services and functionalities as well as the integration of other electronic components within the device. In addition, said radiating system must be cost effective.
Further requirements for radiating systems integrated in wireless handheld or portable devices are focused on minimizing the Specific Absorption Rate (SAR).
Of further importance, usually, is the robustness of the radiating system which means that the radiating system does not change its properties upon smaller shocks to the device.
Owing to the need for the transmission and/or reception of electromagnetic wave signals, a space within the wireless handheld or portable device is dedicated to the integration of a radiating system. The radiating system, and especially the antenna element integrated in the radiating system, is, however, expected to be small in order to occupy as little space as possible within the device, enabling both a size reduction of the wireless device and the integration of additional specific components and functionalities. For instance, it is sometimes particularly convenient to reduce the thickness of the antenna element integrated in the radiating system to enable slimmer devices and/or multiple body devices such as clamshell or slider ones which include two or more parts that can be shifted, folded or twisted against each other. Nevertheless, it is known that there is generally a physical trade-off between the size of a radiating system mainly determined by the size of the antenna element and its performance. That is, in general, a size reduction in for instance the area or thickness of the antenna element is turned into a degradation of its performance.
This is even more critical in the case in which the wireless handheld or portable device is a multifunctional wireless device. Commonly-owned patent applications Publication Nos. WO2008/009391 and US2008/0018543 describe a multifunctional wireless device. The entire disclosure of said applications, Publication Nos. WO2008/009391 and US2008/0018543 are hereby incorporated by reference.
Besides the requirements in terms of acceptable electromagnetic behavior, small size, reduced cost and limited interaction with the human body (such as for instance SAR), other aspects of further relevance when designing a radiating system are those oriented to simplify the manufacturing process. One of the current limitations of the prior-art is that generally the radiating system, namely the antenna system is customized for every particular wireless handheld or portable device platform. The mechanical architecture of each wireless handheld or portable device platform is different and the volume available for the antenna depends to a large extent on the form factor of the wireless handheld or portable device platform and the arrangement of the multiple components embedded into the device (e.g., displays, keyboards, battery, connectors, cameras, flashes, speakers, chipsets, memory devices, etc.). As a result, the antenna within the device is mostly designed ad hoc for every model, resulting in a higher cost and a delayed time to market.
Furthermore, the radiating system integrated in a wireless handheld or portable device must provide enough bandwidth for the emergent applications that require high data rates such as HDTV streaming, video-conference in real time, interactive games, VoIP, etc. However, the bandwidth associated to the cellular communication standards, wireless connectivity standards, and broadcast standards is already allocated and can not be increased mainly due to the well-known electromagnetic spectrum limitations. In this sense, MIMO (Multiple Input Multiple Output) technology appears as a particularly promising solution to increase the data rate required by the aforementioned emergent applications, without the need of increasing said bandwidth. Thus, since it is well-known that in a MIMO system the capacity of the channel is directly proportional to the number of paired antennas (i.e., two antennas in the transmitter (M=2) and two antennas in the receiver (M=2) lead to a MIMO system (M×M) of MIMO order (M) equal to 2, which means that the MIMO system is capable of increasing the channel capacity in a factor around 2 with respect to that provided by a SISO system (Single Input Single Output) composed by a single antenna in the transmitter (M=1) and a single antenna in the receiver (M=1)), MIMO technology is based on the use of multiple antennas in the transmitter and in the receiver in order to attain said desirable data rates. As discussed, the integration of a single multiband antenna capable of providing operation in at least two frequency bands with an acceptable radioelectric behavior in a small wireless device is cumbersome as it is strongly constrained by the physical limitations of the wireless handheld or portable device platforms, so shifting from a single antenna system to a multiple antenna MIMO system becomes challenging.
The prior art solutions disclosed in the literature for providing a wireless handheld or portable device integrating the MIMO technology are usually based on antenna elements with a size comparable to the wavelength of operation (A. A. H. Azremi, M. Kyro, J. Ilvonen, J. Holopainen, S. Ranvier, C. Icheln, P. Vainikainen, “Five-element Inverted-F Antenna Array for MIMO Communications and Radio-finding on Mobile Terminal”, Loughborough Antennas and Propagation Conference, November 2009, Loughborough UK, pp. 557-560; Z. Li, Z. Du, K. Gong, “Compact Reconfigurable Antenna Array for Adaptive MIMO systems”, IEEE Antennas and Wireless Propagation Letters, vol. 8, 2009, pp. 1317-1320). This limitation prevents the possibility of arranging a large number of antenna elements since on one hand the available space in the wireless handheld or portable device is limited and on the other hand undesired coupling effects appear due to the proximity between the antennas elements caused by said limited available space.
Thus, the arrangement of several conventional handset antenna elements in a wireless handheld or portable device in order to provide MIMO capabilities becomes a challenge since usually the antennas will occupy too much space and/or be placed too close to each other. It is known that reducing the size of an antenna results in a penalty on the attainable bandwidth and radiation efficiency, which might severely drop below the minimum required by a particular application, such as cellular communications. In this sense, a trade-off appears since small antennas are preferred for integration in wireless handheld or portable devices incorporating MIMO technology but, at the same time, these elements must provide good radioelectric performance in order to preserve the benefits of the MIMO technology.
Some techniques to miniaturize and/or optimize the multiband behavior of an antenna element have been described in the prior art. However, the radiating structures disclosed therein still rely on exciting a radiation mode on the antenna element (patent application Publication No. US2007/0152886; patent application Publication No. US2008/0042909), thus, setting its size comparable to the operating wavelength.
In this sense, the antenna elements provided by the prior-art (A. A. H. Azremi, M. Kyro, J. Ilvonen, J. Holopainen, S. Ranvier, C. Icheln, P. Vainikainen, “Five-element Inverted-F Antenna Array for MIMO Communications and Radio-finding on Mobile Terminal”, Loughborough Antennas and Propagation Conference, November 2009, Loughborough UK, pp. 557-560; Z. Li, Z. Du, K. Gong, “Compact Reconfigurable Antenna Array for Adaptive MIMO systems”, IEEE Antennas and Wireless Propagation Letters, vol. 8, 2009, pp. 1317-1320) as MIMO solutions for wireless handheld or portable devices mainly operate at a frequency located in a high frequency region where the operating wavelength is small enough to allow the integration of several quarter wavelength antenna elements into the wireless handheld or portable device. Therefore, these proposals are still antenna-based solutions since the contribution to the radiation is predominantly provided by the antenna elements.
Furthermore, a radiating structure operating at a resonant frequency of the antenna element is typically very sensitive to external effects (such as for instance the presence of plastics or dielectric covers that constitute the wireless handheld or portable device), to components of the wireless handheld or portable device (such as for instance, but not limited to, a speaker, a microphone, a connector, a display, a shield can, a vibrating module, a battery, or an electronic module or subsystem) placed either in the vicinity of, or even underneath, the antenna element, and/or to the presence of the user of the wireless handheld or portable device.
Some other attempts (M. Kyro, M. Mustonen, C. Icheln, P. Vainikainen, “Dual-Element Antenna for DVB-H Terminal”, Loughborough Antennas and Propagation Conference, March 2008, Loughborough UK, pp. 265-268; S. K. Chaudhury, H. J. Chaloupka, A. Ziroff, “Novel MIMO Antennas for Mobile Terminals”, Proceedings of the 38th European Microwave Conference, October 2008, Amsterdam The Netherlands, pp. 1751-1754; S. K. Chaudhury, W. L. Schroeder, H. J. Chaloupka, “Multiple Antenna Concept Based on Characteristic Modes of Mobile Phone Chassis”, The Second European Conference on Antennas and Propagation, EuCAP 2007, Edinburgh, pp. 1-6) are focused on antenna elements not requiring a complex geometry while still providing some degree of miniaturization by using an antenna element that is not resonant in the one or more frequency ranges of operation of the wireless handheld or portable device.
The solution presented in (M. Kyrö, M. Mustonen, C. Icheln, P. Vainikainen, “Dual-Element Antenna for DVB-H Terminal”, Loughborough Antennas and Propagation Conference, March 2008, Loughborough UK, pp. 265-268) is based on the aforementioned concept. However, it provides operation in DVB-H and LTE700 communication standards, which are located in a very low frequency region that clearly limits the integration of such antenna elements in wireless handheld or portable devices. Although some miniaturization is achieved, such a solution is not enough to provide low correlation and low coupling or high isolation between these antenna elements.
Owing to such limitations, while the MIMO performance of the former solution may be sufficient for reception of electromagnetic wave signals, the antenna elements still could not provide an adequate MIMO behavior (for example, in terms of input return losses or gain) for a cellular communication standard requiring also the transmission of a significant amount of power in the form of electromagnetic wave signals.
At the same time, those solutions (S. K. Chaudhury, H. J. Chaloupka, A. Ziroff, “Novel MIMO Antennas for Mobile Terminals”, Proceedings of the 38th European Microwave Conference, October 2008, Amsterdam The Netherlands, pp. 1751-1754; S. K. Chaudhury, W. L. Schroeder, H. J. Chaloupka, “Multiple Antenna Concept Based on Characteristic Modes of Mobile Phone Chassis”, The Second European Conference on Antennas and Propagation, EuCAP 2007, Edinburgh, pp. 1-6) providing suitable transmission and reception of electromagnetic wave signals are limited to single band operation.
Consequently, antennas for a MIMO enabled wireless device, such as for instance a mobile phone or handset, need to keep a certain size to fully operate within the entire bandwidth of several frequency bands. Even if a few mid-size antennas fit inside a handset, another challenge is to ensure that the multiple antennas are sufficiently uncoupled and uncorrelated to benefit from the MIMO gain. The challenge further exacerbates when the system has to operate at multiple frequency bands, since the antenna performance strongly depends on the antenna size to wavelength relationship, a fact that clearly makes the achievement of multiband operation in a reduced space even more difficult.
The co-pending patent application Publication No. WO2010/015364, the entire disclosure of which is hereby incorporated by reference, discloses a wireless handheld or portable device not requiring an antenna element for multiband operation. This solution is advantageous since more space is available to integrate other wireless handheld components such as batteries, displays, speakers, front-end modules and the like. Nevertheless, since the ground plane acts as the main radiator, there could appear to be a challenge in providing sufficiently uncorrelated current paths in order to preserve the benefits of the MIMO technology.
As discussed, another limitation of current wireless handheld or portable devices relates to the fact that the design and integration of an antenna element for a radiating structure in a wireless device is typically customized for each device. Different form factors or platforms, or a different distribution of the functional blocks of the device will force to redesign the antenna element and its integration inside the device almost from scratch.
For at least the above reasons, wireless device manufacturers regard the volume dedicated to the integration of the radiating structure, and in particular the antenna element, as being a toll to pay in order to provide wireless capabilities to the wireless handheld or portable device.
In order to solve the aforementioned limitations, this patent application discloses a new solution based on miniature radiation boosters (for example, of the type disclosed in, for example, patent application Publication No. WO2010/015364 referred to above; reference is also made to patent application Publication No. WO2010/015365, relating to an antennaless wireless device using a radiation booster; the entire disclosure of WO2010/015365 is incorporated herein by reference) and their arrangement for MIMO systems inside a wireless handheld or portable device, which benefits from their reduced volume to enable a standardized solution capable of multiband operation suitable for several wireless handheld or portable device platforms.